Method of dispersing exhaust gases

ABSTRACT

A pulse combustion apparatus in which combustion air is brought into heat exchange relationship with the exhaust system of the apparatus for preheating the combustion air prior to entering the combustion chamber. In a preferred embodiment this is achieved by passing the combustion air through a space between an exhaust cushion chamber of the exhaust system and a housing which encloses the chamber.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 654,439 filed Sept. 26, 1984.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of dispersing exhaust gases from apulse combination apparatus or other aspirated fuel burning unit. Forthe purposes of this application "aspirated fuel furning unit" means anapparatus in which fuel is burned in air and combustion gases arepropelled through the apparatus other than by convection. Examples ofaspirated fuel burning units are pulse combustion apparatus and internalcombustion engines, in which the natural cycle of the apparatus propelsthe combustion gases, and boilers or furnaces in which a blower is usedfor this purpose.

2. Description of Prior Art

U.S. patent literature contains numerous examples of prior art pulsecombustion apparatus. Typically, such an apparatus includes a combustionchamber and an exhaust pipe which which forms a resonant system with thecombustion chamber. The apparatus operates on a cycle in which a fuelcharge is admitted to the combustion chamber and ignited. The chargethen expands into the exhaust pipe causing a partial vacuum transient inthe combustion chamber, which both assists in drawing in a fresh fuelcharge and causes high temperature gas to be drawn back into thecombustion chamber from the exhaust pipe. The fresh fuel charge isignited spontaneously from flame fronts in the returning hightemperature gas, thereby establishing the next cycle. Accordingly, theapparatus is self-sustaining after initial ignition. In a pulsecombustion heater, a fluid to be heated is brought into heat exchangerelationship with the exhaust pipe; usually, a heat exchanger isprovided downstream of the exhaust pipe to improve the efficiency ofheat transfer from the combustion gases to the fluid. Exhaust gases aredischarged directly to atmosphere.

Example of pulse combustion heaters are disclosed in my U.S. Pat. Nos.3,267,985, 4,241,720 and 4,241,723. The various forms of apparatusdisclosed are intended primarily (but not exclusively) for use asboilers, i.e. for heating water. However, it has been proposed to usethis type of apparatus specifically for heating air. Examples of thistype of apparatus are shown in my U.S. Pat. Nos. 2,916,032, 4,309,977and 4,336,791.

An object of the present invention is to provide improvements in andrelating to exhaust systems for pulse combustion apparatus and otheraspirated fuel burning units.

BRIEF DESCRIPTION OF INVENTION

One aspect of the invention provides a method of dispersing exhaustgases from an aspirated fuel burning unit having an exhaust outlet, inwhich a perforated dispersion pipe is provided at a location belowground level and outside a building in which the unit is installed. Avent pipe is connected between the exhaust outlet and the dispersionpipe and a bed of particulate material is provided around the pipe andis selected so that the back pressure on the exhaust system is lowerthan the maximum which can be tolerated by the unit. The invention alsoprovides an installation in accordance with the method and an exhaustgas vent system for an aspirated fuel burning unit.

BRIEF DESCRIPTION OF DRAWINGS

In order that the invention may be more clearly understood, referencewill now be made to the accompanying drawings which illustrate a numberof preferred embodiments of the invention by way of example, and inwhich:

FIG. 1 is a side elevational view, partly in section, of a pulsecombustion apparatus suitable for use as an air heater;

FIG. 2 is an elevational view, partly in section, in the direction ofarrow A in FIG. 1;

FIG. 3 is a perspective view of the exhaust cushion chamber of theapparatus of FIGS. 1 and 2;

FIG. 4 is a view similar to part of FIG. 3 showing an alternativeembodiment of the invention.

FIG. 5 is a section view on line V--V of FIG. 2;

FIG. 6 is a diagrammatic elevational view showing a pulse combustioninstallation provided with an exhaust gas vent system in accordance withthe invention; and,

FIG. 7 is a plan view corresponding to FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIGS. 1 to 4, these views show a pulse combustion airheater of the general type described and claimed in my U.S. Pat. Nos.4,309,977 and 4,336,791. The apparatus is generally denoted by referencenumeral 20 and includes a combustion chamber 22, and an exhaust system24 including an exhaust pipe 26, which forms a resonant system with thechamber. Means (to be described) are provided for admitted successivefuel charges to the chamber, and the apparatus also includes ignitionmeans operable to initiate combustion in the chamber, in this caserepresented by a spark plug 30.

The exhaust system includes a manifold 32 by which the exhaust pipe 26is coupled to a heat exchanger 34. The heat exchanger discharges exhaustgases into an exhaust cushion chamber 36 and the gases are exhaustedfrom the system after leaving the chamber 36.

As described in the patents identified above, air to be heated is blownby an external blower (not shown) through duct work indicated in ghostoutline 38, up through the heat exchanger 34 and over the exhaust pipe26 and combustion chamber 22, as indicated by the arrows 40 in FIG. 1.When the pulse combustion apparatus is in operation, the heat producedby combustion in chamber 22 is transferred to this air and the air isthen delivered to the location at which heat is required.

In principle the pulse combustion apparatus itself is constructed andoperates as described in U.S. Pat. Nos. 4,309,977 and 4,336,791, withthe exception that the means for admitting successive fuel charges tothe combustion chamber is designed to bring incoming combustion air intoheat exchange relationship with the exhaust system (in this case by wayof the exhaust cushion chamber 36) for preheating the combustion air aswill be described later. Accordingly, constructional details of thoseparts of the pulse combustion apparatus which are referred to in thesepatents have not been included in the present application and referencemay be made to those patents for such details. The disclosures of bothpatents are incorporated herein by reference.

Design differences in the apparatus of FIGS. 1 and 2 as compared withthe prior patents include a somewhat different configuration for theexhaust pipe 26, designed to improve heat transfer to the airrepresented by arrows 40. Thus, as shown in FIG. 1, the exhaust pipe 26extends generally tangentially from combustion chamber 22 and obliquelyacross the air passageway through which air 40 flows. Heat radiatingfins are formed on the exhaust pipe 26 and combustion chamber 22 forimproved transfer of heat to this air. Some of the fins are individuallydenoted 42a and encircle the exhaust pipe 26 and combustion chamber 22.Manifold 32 connects the single exhaust pipe 26 from combustion chamber22 with a plurality of heat exchanger tubes 44. The tubes 44 allcommunicate with a single transverse passageway 46 in the manifold andexhaust pipe 26 also communicates with that passageway generallycentrally of its length. Again, manifold 32 is provided with a series ofparallel heat radiating fins 48 which in this case are aluminum castingsformed directly on the manifold.

The heat exchanger tubes 34 extend parallel to one another generally ina convoluted S-shaped path from the manifold 32 to the exhaust cushionchamber 36. Aluminum fins on the tubes generally indicated at 50 provideheat radiating surfaces and permit air flow through the heat exchanger.

In FIG. 2, part of the exhaust cushion chamber 36 has been broken awayat line 52 to show end portions of some of the heat exchanger tubesentering the exhaust cushion chamber (as indicated by reference numeral44). Chamber 36 has a single outlet 54 through which exhaust gases leavethe exhaust cushion chamber and the outlet is fitted with a tubularmember 56 which communicates with an exhaust muffler 58 having at itsupper end a main exhaust outlet 60 from the apparatus. Member 56 isdesigned to remove water droplets from exhaust gases leaving chamber 36,as will be described later.

An exhaust cushion chamber in a pulse combustion apparatus receivespulsating exhaust gas waves from the combustion chamber via theremainder of the exhaust system and provides an enlarged space foraccommodating and smoothing out those pulsations before the exhaustgases are discharged from the apparatus. This assists in noiseattenuation. At the same time, moisture within the exhausted gases tendto condense within the exhaust cushion chamber so that the latent heatof vaporization of the moisture is given up before the exhaust gases aredischarged. Nevertheless, in a prior art pulse combustion apparatus,substantial amounts of moisture are exhausted from the apparatus. Oneobjective of the present invention is to promote additional condensationwithin the exhaust cushion chamber and thereby retain within the systemsome of what would otherwise be waste heat. In FIG. 2 a condensate drainfrom the exhaust cushion chamber is shown at 62 and is provided with anexhaust gas trap 63 (to be described later).

Exhaust cushion chamber 36 is shown separate from the remainder of theapparatus in FIG. 3 and it will be seen from that view that the chamberis of generally cylindrical shape. As best seen in FIG. 1, chamber 36 isreceived within a rectangular housing 64 dimensioned to provide a space66 around chamber 36 through which combustion air is conducted forpreheating, when the apparatus is in operation.

Space 66 is, in fact, divided by partitions indicated at 68 in FIGS. 1and 3 to define a spiral or serpentine path for combustion air to flowover the external surface of the exhaust cushion chamber 36. Thepartitions are, in fact, formed by rubber strips which extendlongitudinally of the surface of chamber 36 and are spaced generally by90° angularly about the chamber. Combustion air enters space 66 at aposition indicated at 70 in FIG. 3 adjacent one end of chamber 36. Atthe opposite end of the chamber, the rubber strip 68 at the top andbottom of the chamber are joined by an end portion 68a which effectivelydivides space 66 into two areas at opposite sides of chamber 36. Inoperation, this causes the incoming air to be deflected downwardly andreversed as indicated by the arrows in FIG. 3 at the end of chamber 36remote from inlet 70. The air then travels back towards the inlet but isprevented from mixing with the incoming air by a horizontal partition 72extending between the relevant end of chamber 36 and the externalhousing 64 as best seen in FIG. 2. The direction of flow of thecombustion air is then again reversed and the air flows along the bottomof the far side of chamber 36 as seen in FIG. 3.

Arrows 74 indicate that the air then flows upwardly and back towards theinlet end of space 66. A further partition 76 again prevents mixing ofthat air with the incoming air. An opening 78 (see FIGS. 1 and 2) inhousing 64 permits the combustion air to then leave space 66. Opening 78mates with an air inlet 80 best shown in FIG. 1 to a main housingsection 82 of the apparatus. Combustion chamber 22 has an inletgenerally indicated at 84 which communicates with the interior ofhousing section 82 and the combustion chamber draws its combustion airfrom the interior of the housing section. A blow 86 within section 82 isprovided for starting purposes as described in the patents identifiedabove; after starting, blower 86 is switched off and the air is simplydrawn into combustion chamber 22 through the blower as required by thepulsating cycle of the apparatus.

In this case, the apparatus is designed to operate using gas as a fuel.The gas is delivered through a pipe 88 to a gas cushion chamber 90 fromwhich it is then drawn into the combustion chamber through a series oftubes, two of which are indicated at 92. Combustion air is drawn intothe combustion chamber around tubes 92 and the entry of both air and gas(fuel charges) is controlled by pressure sensitive valves, two of whichare indicated at 94. Again, reference may be had to the patents referredto previously for details of the valve constructions.

In FIG. 2, a cover 96 for the main housing section 92 has been brokenaway to show the blower 86 and the gas cushion chamber 90. Arrows withinhousing section 82 indicate air flowing to the gas cushion chamber 90through blower 86. The blower is designed so that air flow will continueeven when the blower is not operating.

FIG. 2 (and FIG. 1) also illustrate the fact that the main housingsection 82 is supported on the housing 64 for the exhaust cushionchamber 36. The exhaust muffler 58 discussed previously is alsosupported on housing 64 at one side of housing section 82 and an intakemuffler 98 is mounted on housing 64 on the opposite side of housingsection 82. The muffler has a main combustion air inlet at 100 and anoutlet 102 which communicates with an opening housing 64 at thecombustion air inlet position denoted 70 in FIGS. 2 and 3.

It will be apparent from FIG. 1, that the main housing section 82 is ofrectangular shape in plan. The two mufflers 58 and 98 are of similarshape and are of a width (in the plane of FIG. 1) equal to the width ofthe main housing section 82. The two mufflers are shown simply asinternally open chambers although internal baffles or other noiseattenuating devices may be used in practice. On the other hand, at leastinsofar as the intake muffler 98 is concerned, tests have shown that theillustrated design in which combustion air flows around the exhaustcushion chamber also has the effect of attenuating intake noise whichmay even make muffler 98 unnecessary.

When the pulse combustion apparatus is in operation, combustion air willbe drawn in through muffler 98 (initially by blower 86 during startingand later by the combustion cycle in chamber 22). The incoming air willflow in a serpentine or spiral path through the "labyrinth" formedaround the exhaust cushion chamber as best shown in FIG. 3. Oncecombustion has been established and chamber 36 has become heated by theexhaust gases flowing therethrough, heat transfer will take place fromthose gases to the incoming combustion air. The combustion air will beheated while the exhausted gases will be cooled. This latter fact willresult in additional condensation of moisture within chamber 36, (ascompared with an equivalent prior art arrangement) with the result thatthe exhaust gases will give up additional heat to the system. Condensatewill be removed through drain 62. The heated combustion air will thenflow into the main housing section 82 through blower 86 and into thecombustion chamber through inlet 84. The temperature of the combustionair will be raised as compared with an identical system in which thecombustion air is delivered directly to the combustion chamber withoutpreheating. This will mean that the relative humidity of the air will belowered and that the housing and internal components of the apparatuswill be warmed somewhat which will reduce the tendency for moisture tocondense from the combustion air during "off" periods of the pulsecombustion cycle. These factors will reduce corrosion within the mainhousing section 82 and in the combustion chamber and exhaust system. Asa result of preheating combustion air, the temperature of the combustiongases will be raised somewhat (at least upstream of chamber 36), whichwill keep more heat in the system than would otherwise be the case andimprove the efficiency of heat transfer from the combustion gases to theair to be heated.

FIG. 4 shows a modification of the exhaust cushion chamber 36 (denoted36'), in which the external surface of chamber 36 has been formed withlongitudinal corrugations 104 to increase the surface area of thecombustion chamber across which heat transfer can take place, which isbelieved will lead to a further increase in efficiency.

Further improvements as compared with the prior art (including mypatents identified above) are shown in FIG. 2 of the drawings. Theseimprovements include the provision of an exhaust gas trap as indicatedat 63 on the condensate drain from the exhaust cushion chamber and meansfor removing water droplets from exhaust gases leaving that chamber. Asnoted previously, these latter means include a tubular member 56 in theoutlet 54 from the exhaust cushion chamber 36. This tubular member isdisposed in a generally upright position in the chamber and is fitted atits upper end into the lower end of the exhaust muffler 58. The memberhas an open upper end 106 which communicates with the exhaust gas outlet60 of the apparatus. An opening 108 formed in the wall of the tubularmember and forms an inlet through which exhaust gases can flow from theexhaust cushion chamber, into member 56; from there, the exhaust gasescan leave the member 56 through its open upper end 106, and flow intothe exhaust muffler 58. Opening 108 is shaped to promote flow of gasestowards the open upper end of the tube as a vortex so that waterdroplets in the gases tend to collect at the interior surface of thetubular member due to the effect of centrifugal force. This vortex flowis achieved by appropriately shaping opening 108 as best seen in FIG. 5so that incoming gases tend to flow generally tangentially into member56 (as seen in section) and swirl around inside the number adjacent itsinterior surface. In the illustrated embodiment, member 56 is a thinwalled, seamed stainless steel tube and inlet 108 is formed by inwardlydeflecting a portion of the tube adjacent the seam as indicated at 108ain FIG. 5. The length over which this portion of the tube is deflectedis denoted 1 in FIG. 2.

Water droplets which collect at the interior surface of tubular member56, then flow down by gravity towards the lower end of the member. Atransverse baffle 110 extends generally diametrally across the lower endportion of member 56 to break the vortex flow within the member. Themember is positioned over condensate drain 114 so that the droplets canthen flow out of drain 62 and into the exhaust trap 63 along withmoisture which is condensed from the exhaust gases in chamber 36.

An annular member 112 is provided in the upper end of member 56 toinhibit any tendency water droplets on the interior surface of member 56might have to flow upwardly with the exhaust gases.

It is believed that the provision of means within the exhaust system forremoving water droplets from exhaust gases leaving the apparatusrepresents a significant advantage as compared with the prior art inthat it will avoid, or at least reduce emission of water spray from theapparatus. In existing forms of pulse combustion apparatus, it is foundthat a noticeable water spray is emitted with exhaust gases, due to thepresence of significant amounts of water in the gaseous fuels typicallyused. While this is generally not a major problem, under freezingconditions, it can result in the formation of blocks of ice outwardly ofthe exhaust outlet of the apparatus. FIGS. 6 and 7 (to be described)also address this problem.

With continued reference to FIG. 2, it will be seen that the exhaust gastrap 63 comprises a chamber 114 which, in this embodiment, is secured tothe exhaust cushion chamber housing 64 around the condensate drain pipe62 so that liquid from chamber 36, and any exhaust gases which may seepout of pipe 62, will enter chamber 114. Chamber 114 itself has acondensate drain outlet 116 and a float valve 118 is provided in chamber114 and is positioned to close outlet 116 and prevent escape of exhaustgases when the liquid within chamber 114 falls below a predeterminedlevel. This will seal outlet 116 against escape of exhaust gases. Whenthe water level in chamber 114 rises to an extent sufficient to causefloat 118 to rise clear of outlet 116, then the liquid itself will forma seal preventing escape of exhaust gases. Again, in practice, escape ofexhaust gases through the condensate drain in a prior art apparatus isnot regarded as a major problem because the amount of gas that mayescape is quite small. However, the exhaust gas trap 63 does provide anadditional safety feature against any possible danger from this source.

Turning now to FIGS. 6 and 7, these views illustrate diagrammatically aninstallation comprising a pulse combustion heater installed within abuilding, and an improved method of dispersing exhaust gases from theheater. The method not only addresses the problem of moisture emissionsas discussed above, but also aids in dispersing the exhaust gasesthemselves (pollution control), and in attenuating noise. It should beunderstood that this method of dispensing exhaust gases may be used inassociation with any aspirated fuel burning unit and is not limited topulse combustion installations.

As seen in FIG. 6 and 7, the pulse combustion apparatus is generallydenoted by reference numeral 120 and is shown installed inside abuilding having an external wall 122 which is partly below ground level,indicated by reference numeral 124. The apparatus has a combustion airinlet 126 which extends through wall 122 for bringing ambient air to theapparatus 120 for combustion. The apparatus also has an exhaust outlet128 (for example, equivalent to the outlet 60 shown in FIG. 2) and theoutlet is fitted with a vent pipe 130 which also extends through thewall 122 of the building but below ground level. Room air is circulatedthrough the heater as indicated by the arrows X.

A perforated dispersion pipe 132 is provided at a location below groundlevel and the vent pipe 130 is connected to pipe 132 so that exhaustgases and moisture leaving apparatus 120 will flow into the interior ofthe pipe 132 and be dispersed outwardly through its perforations. Pipe132 is buried in a bed 134 of particulate material such as gravel,selected so that the back pressure imposed on the exhaust of theapparatus is below the maximum back pressure which the apparatus cantolerate. In a typical pulse combustion installation using an apparatusin accordance with the invention, or in accordance with one or more ofmy patents identified above, this maximum tolerable back pressure mightbe of the order of one inch water guage. This is due to the relativelystrong cycle of the apparatus but in other forms of apparatus having aweak cycle, the maximum tolerable back pressure would be much lower. Inthe present case, it has been found that a gravel bed at a depth, of sayone foot below ground level, will provide a relatively low back pressure(perhaps of the order of one quarter inch water guage) which is found tohave a negligible effect on the operation of the apparatus.

In any event, it is found that by venting exhaust gases as shown inFIGS. 6 and 7, the moisture within the gases is readily dispersed intothe soil, thereby completely avoiding the spray problem referred toabove, and at the same time the gases themselves disperse through theground and are at least partly absorbed with the result that there isvirtually no noticeable effect at ground level, except, possibly, aslight acidic effect on the soil. In practical tests, this has beenfound to be insignificant and certainly would not be noticeable inalkaline soil areas (e.g. having limestone). In most cases, a beneficialeffect has even been noticed due to the presence of additional amountsof water and carbon dioxide in the soil. In addition to theseadvantages, it has been found that substantial noise attenuation isachieved using the vent arrangement provided by the invention.

If the vent system is used in an area which is prone to flooding, thenit may be quite desirable to provide for vent pipe 130 to be opened toatmosphere in case the dispersion pipe 132 should become flooded whichwould probably prevent starting of the pulse combustion apparatus(although not running once started). As shown in FIG. 6, this isachieved by means of a pipe section extending to a position above groundlevel from vent pipe 130 as indicated in ghost outline at 134. The pipeis provided with a removable cap 136 which could be removed to permitthe pulse combustion apparatus to be started in the event that thedispersion pipe 132 became waterlogged. Pipe 134 may of course bedifferently arranged; for example, it could extend vertically upwardlyfrom the portion of pipe 130 outside wall 122.

The particular form of dispersion pipe 132 and the gravel bed 134 iswhich it is located are not believed to be critical, subject to therequirement that they should not present intolerable back pressure tothe apparatus and the perforations in the pipe should not become pluggedwith time. In a practical installation of a residential size heatingunit, two ten foot lengths of perforated ABS plastic pipe of the typeused for weeping tile were used and one and a half inch PVC pipe wasused for the vent pipe 130. The ends of the dispersion pipe 132 wereclosed by rocks, mainly to prevent plugging of the pipe by the soil butin a commercially manufactured system, the pipe would probably bepermanently capped at its ends. In FIG. 7, the vent pipe 130 is showncoupled to an end portion of the dispersion pipe 132 but again this isnot believed to be critical.

It will, of course, be appreciated that the preceding descriptionrelates to particular preferred embodiments of the invention only andthat many modifications are possible within the broad scope of theinvention.

For example, the preceding description relates specifically to airfurnaces, while it is to be understood that similar principles may beapplied both to boilers and to pulse combustion apparatus for purposesother than heating (e.g. power source) and, in the case of the ventsystem, to other aspirated fuel burning units. It should also be notedthat other arrangements may be envisaged for bringing the incomingcombustion air into heat exchange relationship with the exhaust system.It is believed that the preferred arrangement is to cause the combustionair to flow around the exhaust cushion chamber but a similar effectcould be achieved at other points in the exhaust system. Where heattransfer does take place at the exhaust cushion chamber, it is notessential to employ the serpentine or spiral combustion air flow patterndescribed above although again this is believed to be preferred because,at least in the illustrated embodiment, it results in the formation ofrelatively narrow passageways for the combustion air which causes theair to be accelerated for good heat transfer. The described arrangementof longitudinal partitions in the space around the exhaust cushionchamber allows this to be accomplished in relatively straightforwardmanner and avoids the need to resort to costly expedients such as theformation of spiral vanes or other forms of air passageway aroundchamber 36.

The described means including tubular member 56 for removing waterdroplets from the exhaust, and the exhaust gas trap 63 are optional.Those features can be used individually or in combination in other formsof pulse combustion apparatus, for example, as disclosed in my patentsreferred to above. Also, the vent system of FIGS. 6 and 7 can be usedwith other forms of apparatus. For example, in an apparatus vented inaccordance with FIG. 6 and 7, it probably would be unnecessary toprovide means for removing water droplets from the exhaust gases becausethe water would be dispersed into the soil. In that event, tubularmember 56 could be replaced by a plain short tube simply connection thecushion chamber 36 to the exhaust muffler 58.

I claim:
 1. A method of dispersing exhaust gases from an aspirated fuelburning unit having an exhaust outlet, the method comprising the stepsof:providing a perforated dispersion pipe at a location below the groundand outside a building in which the unit is installed; connecting a ventpipe between said exhaust outlet and said dispersion pipe; and providingaround said dispersion pipe a bed of particulate material selected sothat the back pressure imposed on the exhaust of the unit is less thanthe maximum tolerable back pressure, said bed permitting dispersion intothe ground of exhaust gases leaving said pipe.
 2. An installationcomprising an aspirated fuel burning unit installed within a building,and an exhaust gas vent system comprising:a perforated dispersion pipedisposed at a location below ground level and outside said building; avent pipe connecting an exhaust gas outlet of said unit to saiddispersion pipe; a bed of particulate material surrounding saiddispersion pipe and selected to impose on the exhaust of the unit a backpressure less than the maximum tolerable back pressure, said bedpermitting dispersion into the ground of exhaust gases leaving saidpipe.
 3. An exhaust gas vent system for an aspirated fuel burning unitcomprising a perforated dispersion pipe adapted to be installed in a bedof particulate material at a location below ground level outside abuilding in which the unit is to be installed, and a vent pipe adaptedto couple said dispersion pipe with an exhaust gas outlet of the unit sothat exhaust gases and moisture leaving the unit will be dispersedthrough said dispersion pipe and bed when the unit is in operation, saidbed permitting dispersion into the ground of exhaust gases leaving saidpipe.
 4. An installation as claimed in claim 2, further comprising apipe section provided communication between said vent pipe andatmosphere, above ground level, and removable means normally closingsaid pipe section.
 5. A system as claimed in claim 4, further comprisinga pipe section adapted to provide communication between said vent pipeand atmosphere, above ground level, and removable means normally closingsaid pipe section.
 6. An aspirated fuel burning unit having an exhaustgas vent system comprising:a perforated dispersion pipe disposed at alocation below ground level; a vent pipe connecting an exhaust gasoutlet of said unit to said dispersion pipe; a bed of particulatematerial surrounding said dispersion pipe and selected to impose on theexhaust of the unit a back pressure less than the maximum tolerable backpressure, said bed permitting dispersion into the ground of exhaustgases leaving said pipe.
 7. An installation as claimed in claim 6,further comprising a pipe section providing communication between saidvent pipe and atmosphere, above ground level, and removable meansnormally closing said pipe section.